Up to date, it has not been possible to remove by mechanical means like filters or centrifuges oil from water which is distributed within the water in the form of very small oil droplets having a diameter of about 1 to 30 microns and especially having a diameter below 10 microns. The problem is that such extremely fine oil droplets are suspended in the liquid and dispite their lower density than the density of the liquid, do not or do not within a satisfactory time period rise to the surface of the liquid and especially not if the liquid contains surface active substances. If the oil content of waste water is to be reduced or if stabilized oil-in-water dispersions like coolants for machining metals have to be cleaned from other oils which have contaminated them, it must be possible to separate such fine or extremely fine oil droplets.